1. Field of the Invention
The present invention relates to a walking assistance device which assists a user (person) with walking.
2. Description of the Related Art
Hitherto, as this type of walking assistance device, Japanese Patent Application Laid-Open No. 2007-29633 (hereinafter referred to as “patent document 1”), for example, discloses one proposed by the present applicant. This walking assistance device has a load transmit portion on which a user sits astride, foot-worn portions to be attached to the feet of the user, and leg links which connect the foot-worn portions to the load transmit portion. In this case, each of the leg links is constructed of an upper link member extended from the load transmit portion through the intermediary of a first joint, a lower link member extended from the foot-worn portion through the intermediary of a second joint, and a third joint which bendably connects the upper link member and the lower link member. Further, the third joint is driven by a drive source (actuator) mounted on the upper link member. The third joint is driven to cause load for supporting a part of the weight of the user (an upward translational force) to act on the body trunk of the user through the intermediary of the load transmit portion. Thus, a burden on a leg or legs of the user is reduced.
In the walking assistance device disclosed in the aforesaid patent document 1, the drive source for driving the third joint has an electric motor mounted on an upper portion of the upper link member. The rotational driving force of the electric motor is transmitted to the lower link member through the intermediary of a wire and a pulley, thereby imparting a torque about a joint axis of the third joint to the lower link member. In this case, the torque imparted to the lower link member is proportional to an output torque of the electric motor. Further, the rotational speed of the third joint, that is, the relative rotational speed of the lower link member with respect to the upper link member, is proportional to the rotational speed of the electric motor.
Meanwhile, according to the walking assistance device disclosed in patent document 1, in order to maintain load to be applied to a user from the load transmit portion at target load while the user is walking, it is necessary to increase the torque to be imparted to the lower link member as the flexion degree of the leg link at the third joint increases, that is, as the leg link flexes at the third joint. This is because, as the flexion degree of the leg link increases, the distance between the line of action of load applied to the body trunk of the user from the load transmit portion and the third joint increases accordingly, and consequently, the torque of the third joint required to generate the target load increases.
Thus, according to the conventional walking assistance device disclosed in patent document 1, in order to maintain the load to be applied to the user from the load transmit portion at target load, it has been necessary to increase the output torque of the electric motor as the flexion degree of the leg link increases.
In the conventional walking assistance device, therefore, the maximum value of an output torque required of the electric motor and the variation width of a required output torque (the difference between the maximum value and the minimum value of a required output torque) have been inconveniently large.
Especially in the case where the flexion degree of the leg link at the third joint is small, i.e., when the leg link is stretched or nearly stretched, the sensitivity to a change in the height of the load transmit portion in response to a change in the flexion degree becomes very low, as compared with the case where the flexion degree is relatively large. In other words, as the flexion degree of the leg link decreases, the change amount of the flexion degree of the leg link required to change the height of the load transmit portion by a predetermined very small amount increases. Therefore, in the case where the user attempts to vertically move his/her body trunk especially with a small flexion degree of the leg link at the third joint, it is necessary to change the flexion degree of the leg link relatively promptly in order to vertically move the load transmit portion by promptly following the user's attempted motion. This means that the rotational speed of the third joint required to make the load transmit portion promptly follow the vertical movement of the body trunk of the user (the rotational speed of the lower link member relative to the upper link member) becomes higher as the flexion degree of the leg link at the third joint becomes smaller (as the leg link approaches its stretched state).
Hence, in the conventional walking assistance device, the maximum value of the rotational speed required of the electric motor and the variation width of a required rotational speed (the difference between the maximum value and the minimum value of a required rotational speed) have been inconveniently large.
As described above, the maximum value and variation width of an output torque required of the electric motor and the maximum value and variation width of a required rotational speed thereof are large. This has been making it difficult to reduce the size, weight or cost of a drive mechanism, including the electric motor.